The single-polypeptide RNA polymerase of the T7 bacteriophage has been a model system for studying fundamental mechanisms of transcription ever since it was first identified almost 4 decades ago. Work over the past 15 years has revealed that the functions of this RNA polymerase extend beyond transcription of the phage genes, and that homologues of T7 RNA polymerase are widespread, occurring not only in eukaryotic mitochondria but also in mammalian nuclei, where they form the 4th major class of nuclear RNAPs (spRNAPIV). However, when compared to our detailed understanding of the structure and transcriptional mechanisms of T7RNAP, our understanding of the extra-transcriptional functions of this enzyme or of the mechanisms and biology of its mitochondrial and nuclear homologues is limited. To address these gaps in understanding we will: (1) Define the role of T7RNAP and its regulator T7 lysozyme in recruitment and assembly of the T7 DNA packaging machinery and, using both ensemble and single molecule experiments, describe the molecular details of the T7 DNA packaging reaction , (2) Determine crystal structures of yeast mitochondrial RNAP elongation and initiation complexes, and characterize the mechanism of promoter recognition by this RNA polymerase and of its activation by the mitochondrial transcription factor, (3) Identify the genes regulated by nuclear spRNAPIV and the effects of activation of respiration or of catabolite repression on spRNAPIV activity. These studies will advance our understanding of fundamental mechanisms of macromolecular complex assembly and of transcription processes in the mitochondrial and nuclear compartments of eukaryotic cells.